This invention relates to refrigerated display cases, and more particularly to apparatus for defrosting evaporator coils contained in the display case.
The invention is primarily but not exclusively directed to refrigerated display cases manufactured and sold mainly for use in food stores, such as supermarkets, convenience store outlets, combination food and department stores, hypermarkets, etc.
Commercial refrigeration systems for use in consumer retail outlets of the aforementioned type generally comprise a plurality of display cases, each having its own set of cooling coils and associated air propulsion and control means to move air over, around and/or through the cooling coils and the display case to maintain the desired cold temperature. The cooling coils are connected, in a known manner, by connecting conduits to a centrally located mechanical system including one or more compressors, condensors and associated equipment for supplying liquid refrigerant to the cooling coils in the individual cases.
Periodically, the cases must be defrosted to clear accumulations of ice or snow away from the cooling coils. Three principal types of defrosting techniques are generally employed on commercial refrigeration units. One common type utilizes high wattage electrical heaters adjacent the cooling coils; these heaters are activated during the defrost mode to radiate heat and warm the cooling either directly or by warming the air stream passing over or through the coils. A second type of defrosting means, less common because of its complexity than electric defrost, employs heated refrigerant gas which is circulated through the cooling coils during the defrost mode, usually in the reverse direction to normal refrigeration flow.
Electric defrost and hot gas defrost techniques have commercial disadvantages. Electric defrost requires substantial power consumption and is therefore relatively expensive to operate. Hot gas defrost requires additional valving and conduit connections, greater initial capital outlay and more long term maintenance.
The third main type of defrost technique used increasingly in commercial refrigeration systems employs ambient air circulating through the air flow system during the defrost mode. The ambient air, which is substantially warmer than the refrigerated air, is passed over the cooling coils to warm them sufficiently to defrost them. Air defrost systems in general are known; see for example U.S. Pat. No. 4,144,720 Mar. 20, 1979 to Elmer J. Subera, Melvin W. Steelman and Fayez Abraham and assigned to Tyler Refrigeration Corporation. Air defrost techniques, particularly of the type disclosed in the Subera et al '720 patent, have significant mechanical and operating advantages over hot gas defrost and electric defrost techniques. The air defrost technique does not require the additional piping and valving components of a hot gas defrost system and does not require the high power consuming heater wires of the electric defrost system.
Air defrost utilizes heat extracted from the air in two forms; sensible heat is extracted by dropping the air temperature and latent heat is released through the cooling coil area through the condensation of water vapor in the air. As a rule of thumb, moisture released about 1300 BTUs per pound when condensed. It has been found that under certain store conditions of temperature and humidity, the air defrost method alone may not be completely adequate to defrost the cooling coils of certain types of frozen food cases rapidly enough to prevent partial defrosting and spoilage of food products stored in the case.
In some environmental conditions, the ambient humidity in the store will be relatively low. Under such low humidity conditions, not enough latent heat can be extracted from the air to defrost the cooling coils in a sufficiently short period of time to prevent the ice cream from melting. It has been found that, at such times, air defrost techniques alone are not completely effective to defrost ice cream containing cases without suffering a certain amount of product spoilage due to defrosting and refreezing of the ice cream.
It is desirable to utilize the air defrost technique because, of the three main types of defrost techniques, it is the most energy efficient (as well as having other advantages as noted above). It is also desirable and economically and commercially necessary to defrost refrigerated display cases quickly to prevent undesirable defrosting of the products stored in the display case. This problem has been addressed previously, in U.S. Pat. No. 4,145,893 issued Mar. 27, 1979 to Vogel. The Vogel '893 patent discloses a refrigerated display case having a control circuit for diverting electrical energy from anti-sweat heater wires (which operate during the refrigeration mode) to a cooling coil heater which operates during the defrost mode. According to Vogel, the anti-sweat heater wires, which are generally located adjacent duct outlets or inlets to inhibit condensation collection in those areas, are shut off as soon as the display case control system switches from refrigeration mode to defrost mode. At the same time, the control circuit switches on the cooling coil heater unit to warm air being propelled over and through the cooling coils by the display case fans. Vogel's cooling coil heater unit is always turned on at the start of a defrost cycle and remains on for the entire defrost cycle.
The present invention has certain advantages over the type of defrost technique described in the Vogel '893 patent. The invention described here utilizes a supplemental electric heater and heater control, the operation of which depends only on monitored defrost conditions. The supplemental defrost heater is controlled to operate only when necessary to assist the normal air defrost cycle. If ambient temperature and humidity conditions are such that the defrost cycle will be completed within the required time by air defrost alone, the supplemental defrost heater will not be activated at all. On the other hand, if the heater control detects a low humidity condition at any time during the defrost cycle or detects that the defrost cycle is taking too long, the control will energize the supplemental heater to speed up the defrost cycle only for that portion of the defrost cycle during which the abnormality is detected.